The present invention relates to the calibration of flow sensors, and in particular mass flow sensors of the type employed in flow meters and flow controllers.
In many industrial processes, a notable example of which is semiconductor fabrication procedures, there are many individual process steps which require, for their successful implementation, the supply of one or more gases each at an accurately controlled flow rate, or in an accurately controlled quantity.
When the quantity of a gas must be controlled, this can be done by properly selecting the flow duration of the gas if the flow rate is accurately known. For example, in connection with the manufacture of semiconductors, pyroforic or highly corrosive gases are commonly employed in chemical vapor deposition and etching processes.
In such gas flow systems conventional methods for checking the calibration of the output of a flow sensor are impractical, particularly those methods which involve a flow standard and a source of special gas for calibration.
Other known calibration methods are relatively time consuming and one currently employed alternative to calibration is simply the replacement of one flow meter by another flow meter which has been recently calibrated. This solution has the disadvantage that it requires purging of the gas flow path before replacement of the flow sensor.